Process for producing titanium dioxide pigments

ABSTRACT

TITANIUM DIOXIDE PIGMENT IS TREATED BY MAINTAINING A WATER SLURRY OF THE PIGMENT AT A PH OF BELOW ABOUT 7 AND APPLYING SILICA IN AN AMOUNT OF 3% TO 10% BY WEIGHT, BASED ON THE PIGMENT, BY ADDING A SOLUTION OF SOLUBLE SILICATE TO THE WATER SLURRY. THE RESULTANT SLURRY IS ADJUSTED TO A PH OF FROM 6-8 AND ALUMINA IN AN AMOUNT OF 1% TO 10% BY WEIGHT, BASED ON THE PIGMENT, IS PRECIPITATED IN THE SLURRY. THE PIGMENT PRODUCED BY THIS TREATMENT HAS GOOD HIDING POWER AND FILM INTEGRITY.

United States Patent 3,591,398 PROCESS FOR PRODUCING TITANIUM DIOXIDE PIGMENTS Albert H. Angerman, Wilmington, Del., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Apr. 5, 1968, Ser. No. 719,246 Int. Cl. C09c 1/36, 3/00 US. Cl. 106300 4 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION The treatment of pigmentary titanium dioxide in the rutile or anatase form with hydrous oxides, especially silica and alumina, is known in the art to provide resist ance to chalking and discoloration in paints, good opacity in paper and good hiding power in fiat paints. Typical application techniques for applying silica are taught in US. 2,885,366; 2,387,534; and 2,296,636. Generally, the silica is applied by precipitation from sodium silicate with acid in an aqueous slurry of the pigment. Since the sodium silicate acts as a dispersing agent for the pigment, the art prefers to add the silicate to the slurry prior to addition of the acid, as taught in US. 2,296,636 (page 5, column 9, lines 27-41). A typical technique for applying alumina is taught in US. 2,284,772. Unfortunately, the improved hiding power in fiat paints which these treated pigments possess compared to the untreated pigments, is accompanied by an increased susceptibility of the paint film to staining and its more rapid failure on scrubbing.

For purposes of this application, pigmentary means that the material is pulverulent and of good whiteness with an average particle diameter of from about 0.15 to about 0.3

micron.

SUMMARY OF THE INVENTION In accordance with the present invention a process is provided for treating titanium dioxide pigment by adding to an aqueous slurry of the pigment maintained at a pH of below about 7, a solution of soluble silicate in an amount calculated to provide from about 3 percent to about 10 percent by weight of silica, calculated as SiO based on the weight of the pigment, and thereafter precipitating in the slurry, alumina in an amount from about 1 percent to about 10 percent by weight, calculated as A1 0 based on the weight of the pigment, while maintaining the slurry at a pH of from about 6 to about 8. The process produces a titanium dioxide pigment with good hiding power and film integrity.

DESCRIPTION OF TERMS AND TESTS In the examples which follow, the test results are obtained by the procedures described below.

HIDING POWER (I-LP.)

A film of prepared paint is drawn on a Morest Chart Form 09, a glossy paper chart having black and white areas, using a draw-down blade with an .003 inch clearance. When the film is dried, the refiectances over the white and black backgrounds are measured using a Gard- Patented July 6, 1971 "ice ner Automatic Multipurpose Reflectometer. From these readings the contrast ratio is determined:

reflectance over black reflectance over white Similarly, the contrast ratio is determined on a standard pigment. The relative hiding power of the samples is calculated as:

Contrast ratio H P contrast ratio of sample contrast ratio of standard STAIN RESISTANCE SCRUB RESISTANCE Side-by-side films of a sample and a control are placed on a black chart in the manner described in the Stain Resistance test, and a slurry of 20 parts by weight of water and 50 parts of an abrasive detergent cleansing powder is placed completely over a 2 inch wide strip across both films. A block brush, 3" x 1 /2" x /2" having stiff black butt-cut Chinese hog bristles, is passed back and forth under moderate pressure across the two films. After 20 strokes the chart is rotated 180 and the 20 strokes repeated. This procedure is continued until both films permit the black background to show through. The relative wear of the sample film versus the standard is visually estimated.

The paint for the above procedures is prepared by mixing the following ingredients in a one-quart Cowles Mixer running at 2000 r.p.m. for 3-4 minutes.

Calcium Carbonate, Camel-Tex (H. T. Campbell Co.) Towson, Md. 100.0

After the initial mixing, the speed is increased to 3000 r.p.m. for 10 minutes. The batch is then reduced by mixing in the following:

Gms. Methyl Cellulose, 4000 cps., 3 /2 solution 120.0 Carbitol Acetate (Dow Chemical Company) 120.0

NOPCO NDW 0.5 Polyvinyl Ac. Elvacet (E. I. du Pont de Nemours and Company, Inc.) 224.0 Water 40.0

In both the stain and scrub procedures the rating abbreviations have the following meaning:

sl-: slightly poorer than the standard vsl-: very slightly poorer than the standard vvsl: very, very slightly poorer than the standard =2 equal to the standard better than the standard The standard used in all of the following examples is a commercial titanium dioxide pigment produced by the vapor phase oxidation of TiCl and treated with 1% by weight TiO;, 3% by weight SiO' and 6% by weight A1 The C portions of Examples 4 and are titanium dioxide pigments produced by the seeded hydrolysis of a sulfuric acid solution of ilmenite ore. The standard and C samples are treated as taught in the prior art to add TiO SiO and A1 0 EXAMPLE 1 A 300 grams solid per liter aqueous slurry is prepared by mixing, at 60 C., a rutile pigment obtained from the vapor phase oxidation of TiCl and containing a small amount of A1 0 from the cooxidation of AlCl and water. One-twelfth liter of concentrated HCl is added to 6.67 liters of the slurry (2000 grams of pigment) to 1 efiect a pH of below 4. A solution of sodium silicate, at a concentration of 200 .g./l. SiO and having a weight ratio of SiO /NaO of 3.25, is gradually added during agitation until the pH of the slurry reaches 7. The amount of silicate used corresponds to about 5% SiO on the T basis. Fifty percent sulfuric acid and a sodium aluminate solution containing 120 grams A1 0 are simultaneously added at rates which keep the pH between 6 and 8. The slurry of the pigment with 6% by weight A1 0 is finally adjusted to a pH of 6.8. The pigment is recovered by filtering, washing, drying at about 120 C., and micronizing. The properties of the pigment are shown in Table I.

EXAMPLE 2 To 2497 liters of a slurry (a total of 1100 lbs. of solids) at a concentration of 200 g./l. rutile pigment obtained by vapor phase oxidation of TiCl 15.118 kilograms (33.3 lbs.) of 96% sulfuric acid are added to eflect a pH of below 4. The slurry is warmed to 60 C. and the sodium silicate solution as described in Example 1 is slowly added during agitation until the pH of the slurry reaches about 7. Agitation is continued to insure that the pH is stable and within the 6-8 range. Concentrated sulfuric acid and 85.5 liters of a sodium aluminate solution containing 350 g./l. of A1 0 and a excess of NaOH over the composition NaAlO are added simultaneously in separate properties of the pigment.

Percent Hiding Scrub TiOz SiOz A1203 power resistance 0 6 6 112 v51- 13 1 6 6 111 sl 0 (prior art) 1 3 6 95 vvsl+ EXAMPLE 5 A rutile pigment, prepared as in Example 4 except that it is calcined in the presence of about 2% ZnO, is divided into portions A and B and treated by the methods described in Examples 2 and 3 respectively. The amounts of hydrous oxides and the properties of the pigments are shown in the following table.

Percent Hiding Scrub Sample T102 S102 A1203 power resistance A o 6 e 111 B 1 6 6 105 vvsl- 0 (prior art) 1 3 6 9 vsl- EXAMPLE 6 Another series of tests is run on chloride oxidation process rutile and tabulated below. In each case the amounts of silica and a umina are shown along with relative hiding power, scrub resistance, and stain resistance of the samples prepared by the process of this invention (new) and the prior art method (old) Where sodium silicate is added to the slurry first and then acid is added to neutralize the silicate.

streams to the stirred slurry at rates regulated to hold the pH in the 6-8 range. The amount of reagents used provides 6% SiO;; by weight and 6% A1 0 by weight, based on the pigment. Properties of the recovered pigment are shown in Table I.

EXAMPLE 3 Example 2 is repeated except that, instead of the initial sulfuric acid, 13.9832 kg. (30.81 lbs.) of TiCl are added to the C. pigment slurry with stirring to permit hydrolysis of the T iCl to HCl and hydrous titanium oxide, effecting a pH of below 4. The HCl formed in the slurry is neutralized by the addition of the sodium silicate solution, and the alumina treatment of Example 2 is then applied. The product containing precipitated hydrous oxides equivalent to about 1% TiO 6% SiO and 6% A1 0 is recovered. The resulting properties are shown in Table I.

The base titanium dioxide pigment may be prepared by the high temperature vapor phase oxidation of TiCl the vapor phase hydrolysis of TiCl or the hydrolysis of colloidally seeded sulfuric acid solutions of titaniferous raw materials such as ilmenite. The hydrolysate of the sulfuric acid process must be washed and calcined to develop the crystalline characteristic and particle size needed for the good light scattering characteristics of pigments. The temperature of the slurry being treated may vary from room temperature to 90 C., but it is prefered that the tempeature be from 50 C. to 70 C. The slurry should be agitated throughout the precipitation of both the silica and the alumina to insure general uniformity of the pH conditions within the specified ranges.

THE ACIDIFICATION The slurry must be acid during the addition of the effective portion of the soluble silicate. The acid used may be any acid, such as HCl, H HNO or H PO having a dissociation constant sufiiciently high to precipitate silica, and used in an amount suflicient to maintain an acid condition in the slurry, preferably below a pH of 4, during addition of substantially all the silicate solution. Compounds such as TiOSO or TiCl which hydrolize to form acid, may be used. The acid may be added in excess, but it is preferred that the amount of acid equivalent to the alkali present in a predetermined amount of silicate be added to the slurry prior to introducing the silicate. The silica may also be precipitated at a constant acid level, e.g., pH 1 by the simultaneous addition of the reagents, the optimum acid level being determined experimentally.

THE SILICATE Any soluble silica may be used in the process including sodium or potassium silicate. Commercially available water soluble sodium silicates with SiO /Na O weight ratios from about 1.6 to about 3.75 and varying from 32% to 54% by weight of solids, with or without further dilution, are the most practical. The amount of silica precipitated in the acid environment may be from about 3% to about 10% by Weight, calculated as SiO based on the amount of pigment being treated, but it is preferable that the amount be from about 4% to about 8% by weight with about 6% being the most etfective. Alternative to adding all the acid first, the soluble silicate and the acid may be added simultaneously so long as the acidity of the slurry is maintained at a pH of below about 7.

THE ALUMINA The alumina, essential to good film integrity must be precipitated in an environment having a pH of from about 6 to about 8 to avoid subjecting the treated pigment to unduly acid or alkaline conditions. This is conveniently done by simultaneously mixing an alkaline solution and an acid solution at least one of which contains aluminum, at relative rates which maintain a pH between about 6 and about 8. Illustrative pairs of solutions are sodium hydroxide and aluminum sulfate, sodium aluminate and sulfuric acid and sodium alu-minate and alum. The amount of alumina precipitated may be from 1% to 10% by weight, calculated as A1 based on the pigment being treated, with 4% to 8% by weight being preferred.

This process may be applied to pigments which have been previously treated with oxides such as titania, but their presence is not essential to the benefits herein obtained. After treatment according to this process, the

pigment is recovered by known procedures including neu- 4 tralization of the slurry if necessary, filtration, washing, drying and frequently a dry grinding step such as micronizing. Drying is not necessary, however, as a thick slurry 6 of the product can be used directly in preparing emulsion paints where water is the liquid phase. The process provides a method for obtaining good hiding power and good film integrity in titanium dioxide pigments.

Further variations and applications of this invention will be obvious to one skilled in the art upon reading this disclosure without departing from the inventive concept.

What is claimed is:

1. A process for treating titanium dioxide pigment comprising:

(A) adding to an aqueous slurry of said pigment maintained at a pH below 7, a solution of soluble silicate in an amount calculated to provide from about 3% to about 10% by weight of silica, calculated as SiO based on said pigment; and thereafter (B) precipitating in said slurry, alumina in an amount of from about 1% to about 10% by weight, calculated as A1 0 based on said pigment, while maintaining said slurry at a pH of from about 6 to about 8.

2. A process as in claim 1 wherein said precipitation is accomplished by simultaneously intermixing said slurry with an acid solution and an alkali solution, at least one of which contains aluminum, at a rate controlled to maintain a pH of from about 6 to about 8 during said precipita- Hon.

3. A process as in claim 2 wherein a pH below 4 is initially established by adding an amount of acid stoichiometrically equivalent to said alkali silicate.

4. A process as in claim 3 wherein said pigment is rutile, said acid used to precipitate silica is a sulfuric acid, and said alkali silicate is sodium silicate.

References Cited UNITED STATES PATENTS 7 2,284,772 6/1942 Seidel.

2,296,636 9/ 1942 Hanahan. 2,387,534 10/ 1945 Seidel. 3,035,966 5/1962 Siuta 106-3081 3,418,147 12/1968 Fields 106-308I 3,437,502 4/ 1969 Werner 106-308I 3,146,119 8/1964 Ritter 106-300 3,383,331 5/1968 Allan 106-300 TOBIAS E. LEVOW, Primary Examiner H. M. SNEED, Assistant Examiner US. Cl. X.R. 106-308I 

